Problem with encoding using document function

I don't know why i'm facing this problem only now, cause i'm doing
similar thinhgs allready without problem...

In a styleshhets i have 

     <xsl:apply-templates
select="document('http://tododev/Site/GridHoras/geraXML.asp')"/>

and i'm generating the XML using a Microsoft.XMLDOM document. Actually
i didn't write the code myself, i'm adaapting some code wroted by
someone else.

If i give no encoding in the ASP, i get a error from Saxon
 Invalid byte 2 of 3-byte UTF-8 sequence.	

or similar errors with other processors.

If i include a PI

aDocXml.createProcessingInstruction("xml", " version='1.0'
encoding='ISO-8859-1'")

for some reasing it ignores the encoding and gives the same error.

If i use UTF-16 instead it put's the encoding ok, but now i get 

 Content is not allowed in prolog.	

As i said i've done things like that but only now i'm getting this
errors, go wonder why!!!

What can i do?

Thanks.